Another of the basic properties of sound waves is frequency. Frequency is what we commonly call the pitch or tone of a sound. A higher frequency results in a higher tone while lower frequency results in a lower tone. Within the human hearing spectrum, a piccolo produces higher frequency or higher tones while a tuba produces lower frequency or lower tones. A police siren produces a varying tone. Once we start to talk about ultrasound, all of the frequencies we discuss will be way higher than those of any of the above sources but the relationships remain the same.
Frequency is expressed as the number of complete vibrations or pulses of a sound source per a given unit of time – usually one second. Frequency is normally expressed as the number of cycles (complete waves) per second. “Cycles per second” as a unit of measure has (starting in 1960) been redesignated “Hertz” after German physicist Heinrich Hertz. More information on Hertz, both the man and the unit, is available at wikipedia by clicking here Heinrich Hertz. As the frequency is increased, there will be more complete cycles of a sound wave in a given period of time. Doubling the frequency means that there will be twice as many vibrations or pulses in the same measure of time. The musical term “octave” means doubling the frequency if the note is higher or halving the frequency if the note is lower.
The illustration at the left shows what one cycle of a sound wave looks like when plotted on an X-Y graph and illustrated as a gradient as we have done earlier. The top panel shows a single sound wave of the most simple form, a sine wave. There is one compression (pressure) and one rarefaction (tension) in each “cycle.” The lower panel shows a sound at double the freqency of the upper. Note that are two compressions and two rarefactions in the lower panel in the same period of time. Frequency is infinitely variable and does not need to happen in discreet multiples. If the frequencies represented in the illustration are 1 and 2 Hz, then 4 Hz would have four complete cycles in the same time period. Or, 3.5 Hz would have three and one half cycles in the same time period. Sound frequencies range from Subaudible (below 20 Hz or so) through Audible (20 Hz to 18,000 Hz) and on up through frequencies we call Ultrasonic (20,000 Hz to 100,000,000 Hz) and, finally to Megasonic (100,000,000 Hz and above). Future blogs will explain in more detail properties of sound waves in the Ultrasonic and Megasonic region, but the basic effect of frequency is the same in all cases.
Note – In the preceding paragraph, I used the complete number to describe the various frequencies. Since it is tedious to keep adding zeros for higher frequencies, it is common to use units that include multipliers such as Kilo (times 1,000) and Mega (times 1,000,000) when the numbers get bigger. The symbol for KiloHertz is KHz and the symbol for MegaHertz is MHz. For example, 1 KHz = 1,000 Hz and 2.4 KHz = 2,400 Hz. I will take advantage of these shortcuts in upcoming blogs.
Finally, allthough it is common convention to show the cycle of a sound wave starting at the X axis on and X-Y graph, a wave cycle does not necessarily start where the pressure crosses the X axis or at zero pressure. A “cycle” indicates a repeating pattern and can begin at any point in the cycle as shown below.
In the next blog, I will talk more about the shapes of sound waves, how they travel and reflect from surfaces and discuss some of their interesting properties.
– FJF –